iOS Core notes-multithreading-basic knowledge

1, the basic concept: 1-1, the process:

• Definition: A process is an application that is running in a system.
  
• Understanding: Each process is independent, with each process running in its dedicated and protected memory space.

1-2. Threads:

• To perform a task, a process must have a thread (that is, each process has at least one thread, which is the main thread).
  
• All tasks in a process are executed in the thread.
  
• A thread is the basic execution unit of a process.

1-3, Thread of serial:

• All tasks in a thread are executed serially, i.e., if multiple tasks are to be performed in 1 threads, more than one task can be executed sequentially, and 1 threads can only perform a single task at the same time.

1-4. The difference between a process and a thread:

?了解: The ① thread is the smallest unit of CPU execution tasks;

?了解: ② process is the basic unit of CPU allocating resources and dispatching;

?了解: ③ can have multiple threads in a process, and a process has at least one thread (main thread);

?了解: ④ a thread in the same process to share the resources of the process.

2, Multithreading: 2-1, Multithreading Introduction:

?重要：1, a process can open multiple threads, each thread can perform different tasks in parallel.

?重要：2, multithreading technology can improve the execution efficiency of the program to a certain extent.

as shown in the following:

2-2, Multithreading principle:

?重要：1, at the same time, the CPU can only handle one thread, only one thread is working (execution);

?重要：2, multi-threaded concurrent execution, the essence of the CPU is fast in the scheduling between multiple threads, switching execution, that is: If the CPU Scheduler thread fast enough, it can cause multi-threaded concurrent execution of the illusion.

?备注：Think: What happens if you have too many threads?
答案: ①CPU busy scheduling, switching between n threads, consuming a lot of CPU resources, ② the frequency at which each thread is scheduled to execute is reduced (that is, the execution of the thread is less efficient).

2-3, multithreading advantages and disadvantages: 1, multi-Threading benefits:

?重要：① can improve the execution efficiency of the procedure properly;

?重要：② can appropriately improve resource utilization (CPU, memory utilization).

2. Multithreading Disadvantages:

?重要：① the cost of creating a thread, the main costs under iOS include: Kernel data structure (approximately 1KB), stack space (sub-thread 512KB, main thread 1MB, can also use-setstacksize: set, but must be a multiple of 4KB, and the minimum is 16KB), Creating a thread takes approximately 90 milliseconds to create the time;

Line Stacks Space:

To set the line stacks space:

?重要：② If a large number of threads are turned on, the performance of the program is reduced, and as the thread increases, the overhead of the CPU on the dispatch thread increases.

?重要：The more ③ threads, the more complex the programming (for example: communication between threads, data sharing among multiple threads).

3, the main thread: 3-1, the main thread introduction:

?了解：After an iOS program runs, 1 threads are turned on by default, which is called the "main thread" or the UI thread.

3-2. Main thread:

?重要：1, display, refresh UI interface;

?重要：2. Handle UI events (for example: Click events, scrolling events, drag events, etc.).

3-3, the main thread of use note:

?重要：1, do not put the time-consuming operation in the main thread to execute;

?重要：2, time-consuming operation will be stuck in the main thread, seriously affect the smoothness of UI refresh, user experience is very poor.

3-4, how to determine whether the thread is the main path:

?了解：① View the number = = 1 of the thread or name = = Main;

?了解：Ismainthread method of ②nsthread class method;

?了解：③nsthread Object method Ismainthread method.

4. Multi-threaded implementation in iOS:

iOS Core notes-multithreading-basic knowledge